Recently, transparent electrodes have been widely applied with diversification and development of various kinds of portable electronic devices, eco-friendly energy devices such as solar cells, and displays. An energy device needs a transparent electrode because it is required to allow movement of electrons while transmitting light, and transparent electrodes are indispensable in a touchscreen for, for example, a smart phone.
Currently, indium tin oxide (ITO) is the most widely used material of transparent electrodes. The thin film of ITO has high light transmittance and excellent conductivity as an electrode. However, as the demand for ITO increases, there is a growing concern about the amount of indium tin oxide present on the earth. Moreover, there is a problem that the output of indium, which is the core of indium tin oxide, is almost dependent on one country, China. In addition, ITO has a natural ceramic characteristic, which makes it difficult to apply ITO to flexible electric devices. Accordingly, various studies are under way to develop a material that can replace ITO as a transparent electrode.
As nanotechnology evolved, nanomaterials that did not exist or were uncontrolled have been synthesized. Among these nanomaterials, nanocrystals, carbon nanotubes, silicon nanowires, and metal nanowires are being applied to electronic devices. Particularly, the metal nanowires are advantageous in that they can be applied to transparent electrodes because they have high conductivity and are so small that they are indistinguishable in the visible light region.
In particular, silver (Ag) has not only high electrical conductivity and thermal conductivity but also high-efficiency surface-enhanced Raman effect in the visible light region among other metals, and thus has excellent optical properties. When silver (Ag) is prepared in the form of nanowires, it may be applied to various fields ranging from microelectronic devices to transparent electrodes, are also expected to be used as optical, chemical or biosensors. However, in order for silver nanowires to be used in various fields, mass production of silver nanowires with a uniform thickness, uniform size, clean surface, high aspect ratio, small size deviation is of the greatest importance.
However, silver nanowires have a disadvantage in that growth thereof in the longitudinal direction is limited. According to percolation theory, the longer a linear substance in a certain area is, the smaller the population used to have conductivity is. Such decrease in the population may increase the transmittance, thereby increasing the transparency of the electrode.
Copper is a low-cost material that exhibits as high conductivity as gold or silver. However, unlike gold or silver, copper is easily oxidized in the atmosphere. If copper is divided into nano-size pieces, the surface area increases and thus the copper pieces are more easily oxidized. Therefore, employing copper easily oxidizable in the atmosphere in industries is not possible or requires a very expensive and complex process.
Particularly, as the technical requirements for flexible electronic devices recently increased, there has been a demand for a highly durable and extensible electrode. Among conventional methods of manufacturing an extensible electrode, there is a method of manufacturing a chain-like microstructure using metal nanowires. In this method, an expensive oxidation-stable material such as silver is used to manufacture the microstructure.
However, this method has a disadvantage in that the wires have low conductivity due to weak connectivity therebetween, and fine patterning is not easy. As a method for improving selectivity and conductivity, there is a method of locally sintering silver nanowires using laser, but this method is limited since it requires fabrication of expensive silver nanowires.
Korean Patent Application Publication No. 10-2005-0086693 (entitled “DISPERSION OF NANOWIRES OF SEMICONDUCTOR MATERIAL”) discloses a method of manufacturing nanostructures having a substantially uniform length within a given error margin.
Korean Patent Application Publication No. 10-2005-0079784 (entitled “MICROWAVE POWER AMPLIFIER USING COPPER OXIDE NANOWIRE OR COPPER NANOWIRE AS A CATHOD AND MICROWAVE POWERAMPLIFIER MODULE USING THE SAME”) discloses a microwave power device including a cathode, a gate, and anode, wherein the cathode is formed of copper oxide nanowires formed by contacting a copper surface with an oxidizing solution at a low temperature below 100° C.
However, since this nanowire manufacturing method does not include a separate process for controlling the oxidation characteristics of copper, it is difficult to control the oxidation characteristics of copper nanowires with this method.